This prospective, single-center, observational study aims to evaluate changes in the frontal QRS axis on ECG before and after spinal anesthesia in patients undergoing total knee arthroplasty. The primary objective is to determine whether spinal anesthesia causes a significant alteration in the frontal QRS axis. A total of 80 patients will be enrolled, and preoperative and postoperative ECG measurements will be compared using paired t-test or Wilcoxon signed-rank test depending on data distribution.

This study will be conducted in the Department of Anesthesiology and Reanimation at Elazığ Fethi Sekin City Hospital. It is a prospective, observational, and single-center study designed to evaluate the impact of spinal anesthesia on the frontal QRS axis in patients undergoing elective total knee arthroplasty.

The primary endpoint is the difference between preoperative and postoperative frontal QRS axis measurements. ECG recordings will be taken within 2 hours before and after surgery. Frontal QRS axis values will be automatically calculated by the ECG machine and verified manually.

Inclusion criteria include patients aged 45-85 years, ASA class I-III, undergoing spinal anesthesia for elective knee replacement surgery, and able to obtain ECG recordings before and after the procedure. Patients with arrhythmias such as atrial fibrillation or pacemaker dependency, or those requiring postoperative ICU care or general anesthesia conversion will be excluded.

The estimated sample size was calculated using G*Power software. Based on previous studies, a minimum of 34 patients is required to detect a clinically significant change of 10 degrees in QRS axis, with a planned enrollment of 80 patients considering data loss.

Statistical analysis will be conducted using SPSS 26.0. A p-value <0.05 will be considered statistically significant.

Inclusion Criteria:

• Aged between 45 and 85 years
• ASA physical status I-III
• Scheduled for elective total knee arthroplasty under spinal anesthesia
• Able to tolerate 12-lead ECG before and within 2 hours after surgery

Exclusion Criteria:

• Known rhythm disorders affecting ECG axis (e.g., heart block, atrial fibrillation, pacemaker)
• Conversion to general anesthesia
• Admission to intensive care unit after surgery
• Missing ECG data
• Communication or mental disorders

Patients aged 45-85 years undergoing elective total knee arthroplasty under spinal anesthesia at Elazığ Fethi Sekin City Hospital.